Assessment of titanosilicate glasses for the vitrification of nuclear waste streams containing high concentrations of sodium

Assessment of titanosilicate glasses for the vitrification of nuclear waste streams containing high concentrations of sodium

Mike T Harrison
<mike.t.harrison@nnl.co.uk>

Mike Harrison is a technical specialist with over 10 years’ experience in the vitrification of a wide range of radioactive waste streams, including HLW, ILW, plutonium and uranium. He is technical lead of vitrified wasteform performance activities, and has developed considerable knowledge of glass dissolution mechanisms and durability testing methodologies, including a wide range of ASTM standards. His work also includes glass formulation chemistry, vitrification processes and wasteform properties.

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Sodium carbonate is currently being considered as a wash-out reagent for the removal of the settled solids in the unagitated Highly Active Liquor (HAL) storage tanks at Sellafield. As the settled solids are expected to comprise mainly zirconium molybdate (ZM), this will result in a feed to the Waste Vitrification Plant (WVP) containing high concentrations of both molybdenum and sodium. This feed is expected to be challenging for WVP due to solubility limitations in existing base glass compositions and the operating temperature.

In particular, depending on the wash-out strategy employed, there is the potential to produce high volumes of sodium-containing waste, which will result in large quantities of vitrified HLW containers. Hence, as part of a collaborative Innovate UK-funded project, high sodium titanosilicate (NTS) glass compositions are being investigated in order to maximise the waste content of the vitrified product and minimise the number of containers produced.

Following small-scale experimental trials to determine the optimum concentrations of alumina (Al2O3) and boria (B2O3) to add to NTS glasses, two new base glasses were produced for product quality testing with a high-Na ZM/CPM waste simulant. These compositions minimised the phase separation whilst maintaining acceptable chemical durability. For these hybrid ‘NABTS’ glasses, the remainder of the measured product quality (PQ) parameters were comparable to (or better than) those for standard HLW product glasses. The exception was thermal stability, which was found to be worse, although due to the expected lack of fission products in the Oldside wash-out waste stream this is not expected to be a major issue.

However, glass melt corrosion trials at 1050 °C revealed an extremely high corrosion rate of the melter crucible material (Nicrofer® 6025HT) rendering this composition unusable on WVP. However, initial trials showed that the addition of ~2 %w/w Cr2O3 to the glass significantly reduced the corrosion to acceptable values. Hence, modifying the NABTS glasses with chromium may allow WVP operations to tolerate this composition, although further underpinning work would be required.